A router or switch (network device) directs packets from an input to an output. The classic network device architecture includes multiple line cards having onboard queue managers, that connect to a central switching mechanism. FIG. 1 is a high level depiction of a network device 10 depicting line cards 12 connected to a crossbar switch 14 by serial links 16. The crossbar includes crossbar switches in the form of ASICs coupled to a backplane formed of multiple serial links. The line cards act as the inputs and outputs of the router and are coupled to a backplane switching fabric which performs the task of coupling an input line card to an output line card for data transfer. Traffic from the input line card is passed through a switch the backplane switching fabric and passed to the proper output line card. High speed network device applications utilize a serial backplane where data is transferred on high-speed serial paths formed in the backplane.
The queue managers must implement flow control to prevent data from being lost when buffers on a receiver overflow. Backpressure is used to control flow and the receiver has the ability to prevent transmission of data until its buffers have capacity to receive more data.
At the transmitter, head of line (HOL) blocking may occur when a packet at the head of the transmit queue can't be transmitted because of backpressure. This blocks all packets in the transmit queue from being transmitted even if their intended receiver is available.
The problem of HOL blocking is solved by forming virtual output queues (VOQs) on the line cards where a VOQ is established for each potential receiver. Thus, if the head packet on a VOQ is blocked the line card can switch to another VOQ having a head packet that is not blocked.
The backplane interconnect architecture is a key chokepoint in today's communication infrastructure designs. A great deal of attention has been paid to moving data between chips on a board and there has been growing acceptance of Gigabit and Multi-Gigabit Ethernet as a point to point switched interconnect technology.
Flow Control is a process that is found in most Ethernet networks. It is needed to ensure that devices do not overload other devices on the network. Special frames called ‘PAUSE frames’ are used to signal traffic flow requests and status between nodes. PAUSE frames permit one end station to temporarily stop all traffic from the other end station (except MAC Control frames).
For example, assume a full-duplex link that connects two devices called “Station A” and “Station B”. Suppose Station A transmits frames at a rate that causes Station B to enter into a state of congestion (i.e. no buffer space remaining to receive additional frames). Station B may transmit a PAUSE frame to Station A requesting that Station A stop transmitting frames for a specified period of time. Upon receiving the PAUSE frame, Station A will suspend further frame transmission until the specified time period has elapsed. This will allow Station B time to recover from the congestion state. At the end of the specified time period, Station A will resume normal transmission of frames.
Note that the PAUSE frame protocol is bi-directional. Station A may send frames to pause Station B, and Station B may send frames to pause Station A. A PAUSE frame is the one type of frame that a station is allowed to send even if it is currently in the paused state. Support for PAUSE frames is optional among devices that implement the full-duplex protocol (the use of PAUSE frames is not supported in a half-duplex environment). It is valid for a device to support only half of the protocol; i.e. it may transmit PAUSE frames without having the capability to decode them on the receive side, and vice-versa. Devices use the Auto-Negotiation protocol to discover the PAUSE frame capabilities of the device at the other end of the link. This permits interoperability between devices that do or do not support one or both halves of the protocol.
However, the current flow control using PAUSE frame does not fit very well in the backplane architecture as it does not distinguish internal queues of the system and prevents traffic to flow between unclogged queues when one of the queues become full.
The problem with PAUSE frames is that the transmitting device stops all further transmissions for duration of the PAUSE event in units of 512-bit times. This works very well for devices connected over the Ethernet network in a LAN/WAN environment. However, this can become a Quality of Service (QoS) bottleneck in a backplane interconnect environment.
The challenges in the field of backplane interconnect design continue to increase with demands for more and better techniques having greater flexibility and adaptability. Therefore, a need has arisen for a new system and method for providing backpressure control in Gigabit Ethernet systems allowing for QoS and Class of Service (CoS) requirements to be met.